1. Field of Invention
The present invention is generally directed toward a battery powered implantable bone growth stimulator and more specifically to a threaded screw made of nonconductive material upon which a hermetically sealed battery casing is mounted to provide electrical stimulation for bone growth.
2. Background of the Invention
The present invention is directed toward the electronic stimulation of bone (osteogenesis) through or around an orthopedic bone fixation device with an attached implantable bone growth stimulator. It has long been known that the application of electric currents (electric stimulation) can speed bone growth and healing. The electronic stimulation of bone growth has been used in the treatment of fractures, nonunion of bone and to hasten rates of bone fusion as early as the 1800's. Yasuda, in the 1950's in Japan studied the effect of electricity in the treatment of fractures. E. Fukudain “On the piezoelectric effect of bone”, J Physiol. Soc. Jpn. 12:1158-62, 1957, and Yasuda, J. Kyoto Med. Assoc. 4: 395-406, 1953 and showed that electric signals could enhance fracture healing. Both direct current capacitively coupled electric fields and alternately pulsed electro magnetic fields affect bone cell activity in living bone tissue.
Bone has bioelectrical properties with naturally occurring generated stress potentials. When the bone is stressed, it will carry an electropositive charge on the convex side and an electronegative charge on the concave side. Wolff's Law demonstrates that bone will form new bone in areas of compression and bone will be resorbed in areas of tension. This biological response to stress in bone creates mechanically generated electrical fields or “strain related potentials. Areas of active growth in bones carry an electronegative charge. When a bone fractures, the bone becomes electronegative at the fracture site. On a cellular basis it has been discovered that osteoblasts are activated by electronegative charges. Research on the effects of electrical forces on bone cells in bone formation and healing has demonstrated that bone healing can be hastened and enhanced by electricity. Studies have shown that by implanting an electrical stimulation device and applying an electrical current around the bone, that bone formation is increased around the cathode (negative electrode) and decreased around the anode (positive electrode). Further research of the use of bone growth stimulators has discovered that the optimal current for bone growth with electrical stimulation is believed to be between 5 and 20 micro amperes.
K. S. McLeod and C. T. Rubin in “The effect of low frequency electrical fields on osteogenesis”, J. Bone Joint Surg. 74a:920-929, 1992, used sinusoidal varying fields to stimulate bone remodeling. They found that extremely low frequency sinusoidal electric fields (smaller than 150 Hz) were effective in preventing bone loss and inducing bone formation. They also found strong frequency selectivity in the range of 15-30 Hz. Fitzsimmons et al. in “Frequency dependence of increased cell proliferation”, J Cell Physiol. 139(3):586-91, 1985, also found a frequency specific increase in osteogenic cell proliferation at 14-16 Hz.
U.S. Pat. No. 5,292,252 issued Mar. 8, 1994. discloses a stimulator healing cap powered by an internal small battery. The cap can be reversibly attached to a dental implant, and stimulates bone growth and tissue healing by application of a direct current path or electromagnetic field in the vicinity of bone tissue surrounding the implant, after the implant is surgically inserted.
Another dental device described in U.S. Pat. No. 4,027,392 issued Jun. 7, 1972 discloses an embodiment of a bionic tooth powered by a battery including an AC circuit. The microcircuitry indicated by its FIG. 3 is not shown as being incorporated within the cap.
Another related device is disclosed by in U.S. Pat. No. 5,738,521 issued Apr. 14, 1998 which describes a method for accelerating osteointegration of metal bone implants using AC electrical stimulation, with a preferably symmetrical 20 mu·A rms, 60 KHz alternating current signal powered by a small 1.5 V battery. However, this system is not a compact, self-powered stimulator cap, but is externally wired and powered.
Osteogenetic devices are as described in U.S. Pat. No. 6,605,089 issued Aug. 12, 2003 which discloses a self contained implant having a surgically implantable, renewable power supply and related control circuitry for delivering electrical current directly to an implant which is surgically implanted within the intervertebral space between two adjacent vertebrae. Electrical current is delivered directly to the implant and thus directly to the area in which the promotion of bone growth is desired.
U.S. Pat. No. 6,034,295 issued Mar. 7, 2000 discloses an implantable device with a biocompatible body having at least one interior cavity that communicates through at least one opening with the surrounding body so that tissue surrounding the implantable device can grow through the opening. Two or more electrodes are contained within the device having terminals for supplying a low-frequency electrical alternating voltage and at least one of which is located inside the cavity. U.S. Pat. No. 5,030,236 issued Jul. 9, 1991 also discloses the use of electrical energy that relies upon radio frequency energy coupled inductively into an implanted coil to provide therapeutic energy. However, none of these devices perform satisfactory osteogenesis promotion, while leaving the implant member or stem essentially unchanged in appearance and mechanical properties.
The art that relates specifically to bone growth stimulation by small, self powered electrical means is very limited and most of the bone graft stimulation has been undertaken using power sources located outside the patient's body. Another problem that occurs when the implant is self powered is that the power short circuits against the metal screw or device.
There is thus a widely recognized need for a practical, self-powered osteogenesis implant that can generate electrical stimulation signals. It would also be extremely advantageous that such implants, when used for example in hip or knee implants, should require minimal changes to both appearance and mechanical integrity and function of the implants. The primary goal of such devices would be to increase bone density and implant bone contact ratio around any new implant as a routine common clinical practice.